A swarm of earthquakes, including a magnitude 6.0 and two greater than magnitude 5.0, have hit Christchurch, toppling already damaged buildings, injuring residents and disrupting power, phone services and retailers.

The magnitude-6.0 quake, which struck at 3.18pm, was six kilometres deep and centred 10km north of Lyttelton, Geonet said.

It was larger than the magnitude-5.8 tremor that shook the city at 1.58pm, causing widespread panic and damage.

A 5.3 quake hit at 2.06pm. It struck 20km east of Belfast at a depth of 10km.

Part of a Scarborough rockface collapsed, but the rock was held by shipping containers at the bottom of the cliff.

Police say there is no tsunami threat and no reports of serious or widespread damage or injury.

Damage has been reported to the Work and Income building in New Brighton, with all staff safely evacuated.

A stopbank in Kaiapoi is damaged and the India Blue restaurant in Sumner is damaged and has been cordoned off.

A building on the corner of Charles St and William St in Kaiapoi is at risk of collapse.

Police are urging people to stay away from hill suburbs because of risk of rockfalls.

Police said all available units had been dispatched across the city to provide reassurance and check on safety and damage, the spokesman said.

The Lyttelton tunnel is open.

Civil Defence's national crisis management centre in Wellington has been activated.

Canterbury Earthquake Recovery Authority (Cera) chief executive Roger Sutton said he had flown around the city and saw liquefaction similar to the September quake.

AIRPORT OPEN

Christchurch International Airport chief executive Jim Boult said flights would restart at 6pm.

The airport was closed for engineering checks after the first quake. Some flights were diverted and cancelled.

POWER AND PHONES OUT

Orion chief executive Rob Jamieson said this evening that power was now back on in rural areas, but about 19,000 customers were still without power in the eastern suburbs.

They hoped most of the outages were caused by "tripping" and not cable damage, he said.

Phone networks are down and Telecom has asked residents to keep mobile calls for emergency use where possible, and use text messages.

NO CHRISTMAS EVE MAIL

There will be no mail deliveries in the Christchurch area by New Zealand Post tomorrow.

The Christchurch Mail Centre has been evacuated as a precaution for the safety of staff and will not reopen for processing tonight.

The next scheduled New Zealand Post mail deliveries following the statutory holidays will be December 28.

Post shops will be open tomorrow on a case-by-case basis at the discretion of individual store managers.

COUNCIL BUILDINGS CLOSED

All council buildings have been closed until they can be assessed as safe to occupy.

Christchurch Mayor Bob Parker, who was holidaying in Taupo, was trying to get back to the city.

He said the quake was another setback physically and psychologically.

"Coming almost on Christmas Eve, with all the shops shutting, malls evacuated and airports closed, and people sitting around in tears ... you just can't underestimate the ongoing stress this has created for so many people,'' he told Television One.

"It's just going to be one hell of a day and my heart just breaks really to think so many people have already been so much.''

DISTRICT HEALTH BOARD

Burwood Hospital is operating on emergency power and the birthing unit at Burwood Hospital has been temporarily relocated. Mothers who go into labour and had planned on giving birth at Burwood are asked to attend Christchurch Women's Hospital instead.

Christchurch City Council have reported that some sewage discharge to the Heathcote estuary has occurred. Residents are advised to avoid contact with the estuary until at least 4pm on Sunday, providing there are no further aftershocks.